Method for removing the oxides of nitrogen as air contaminants

ABSTRACT

A method for treating a stream of combustion effluents, containing NOx and other contaminants, said method comprising the steps of first contacting the stream with a sufficient amount of hydrocarbon in the presence of oxygen at a temperature ranging from 400 to 2,700*C. for sufficient time to substantially reduce the NOx to molecular nitrogen; contacting the reduced stream with sufficient oxygen at a temperature ranging from 400 to 2,700*C. for a time sufficient to substantially oxidize all other contaminants to an oxidized state. It has been found that this reduction can also be carried out in an intermittent manner in which the reducing hydrocarbon-oxygen mixture can be shut off for hours at a time with continuing NO- removal (the self-contained reductant mode).

United States Patent Myerson Feb. 18, 1975 [5 METHOD FOR REMOVING THE OXIDES 3,476,508 11/1969 Kearby et a1. 423/2132 01. NITROGEN AS AIR CONTAMINANTS 3,656,915 4/1972 Tourtellotte 423/2132 x 3,696,618 10/1972 Boyd et al 423/213.2X [75] Inventor: Albert L. Myerson, Mounta1ns1de,

Primary Examiner-GO. Peters [73] Assignee: Exxon Research and Engineering Attorney Agent, or Firm-John Paul Cowman Company, Linden, NJ. 22 Filed; Mar. 16, 1973 L th d f ABSTRACT f b m me 0 or treatmg a stream 0 com us 1on e u- [211 Appl- 341,842 ents, containing NO, and other contaminants, said R l U S A fi ti Data method comprising the steps of first contacting the [63] continuatiommpan of Ser No 246 841 Apr 25 stream with a sufficient amount of hydrocarbon in the 1972, abandoned presence of oxygen at a temperature ranging from 400 to 2,700C. for sufficient time to substantially reduce [52] U.S. c1 423/212, 423/235, 60/274, the N01 to molecular nitrogen; contacting the 60/303 duced stream with sufficient oxygen at a temperature 51] 1111. C1 BOld 53/34 ranging from 400 to for a time Sufficient [58] Field of Search 423/212, 2132, 2135, substantially oxidize all other contaminants to an oxi- 423/239, 351 60/274, 303 dized state. It has been found that this reduction can also be carried out in an intermittent manner in which {56] References Cited the reducing hydrocarbon-oxygen mixture can be shut UNITED STATES PATENTS off for hours at a time with continuing NO- removal (the self-contained reductant mode). 1,154,171 8/1915 Brownlee et al. 423/351 3,370,914 2/1968 Qrgssgt 423 2132 7 Claims, 3 a ng gu s PATENTED FEB I 81973 SHEET 10F 3 PATENTEB FEB I 8 i975 SHEET 3 BF 3 We zwwta 535m on Qw o O U], U 65 5 5 =65 oil wdd (ON) d (NOH) 1 METHOD FOR REMOVING THE OXIDES OF NITROGEN AS AIR CONTAMINANTS This application is a continuation-in-part of an application filed Apr. 25, 1972 bearing Ser. No. 246,841 now abandoned.

This invention relates to the treatment of combustion effluent and waste products in a multiple zone system in a manner which renders them less objectionable for discharge to the atmosphere.

In one aspect, this invention relates to the chemical reduction of NO, by hydrocarbons and oxygen at elevated temperatures.

In another aspect, this invention relates to the chemical reduction of NO in a noncatalytic manner.

The oxides of nitrogen referred to collectively as NO constitute a family of air contaminants. The most important ones are listed as follows:

Of these, N can be eliminated from consideration as an air contaminant. Emission sources are rare and it has no effect whatever at low concentrations. At a concentration of 30 percent or so, it is useful as an anesthetic. N 0, is a polymer of N0 which forms when the N0 is compressed and liquefied. When N0 is diluted to any extent with air, N 0 does not exist in significant amounts.

Combustion effluent and waste products from various sources are discharged into the atmosphere. Unless the waste products are treated to remove deleterious components, the waste products may contaminate the atmosphere. Much of the N0 contamination comes from automobiles and fuel-burning processes. However, chemical and manufacturing processes and power plants contribute to the problem and also cause such nuisances. Most of the oxides of nitrogen are produced by oxidizing fuels during which a part of the oxygen combines with atmospheric nitrogen in the flame rather than with the fuel. This process is called nitrogen fixation and occurs in flames. specific examples are exhaust products from automotive engines and combustion effluents from stationary sources such as power plants and industrial boilers. Such waste products contain one or more components such as unburned hydrocarbon, carbon monoxide, nitrogen oxides, sulfur compounds, partially oxidized products, etc., in various concentrations. Part or all of these are believed to contribute to the smog problem presently facing various geographical areas of the United States and other countries.

It is believed that these combustion products react with oxygen under the influence of sunlight to produce smog. With the growing number of automobiles, particularly in urban areas, the discharge of such waste products in the atmosphere will further increase and aggravate the problem.

Methods for converting vehicular exhaust gases to less harmful material, such as carbon dioxide, nitrogen and water vapor may be either catalytic or of the thermal conversion type. In the catalytic method, the exhaust gases are passed into contact with a suitable conversion catalyst prior to discharge in the atmosphere. In the thermal method for hydrocarbon and CO removal, the exhaust gases are heated to or are maintained at a sufficiently high temperature in the presence of oxygen to burn without utilization of the catalyst but this does not remove nitric oxide. With either method, it is necessary to provide a stoichiometric amount or preferably an excess amount of oxygen or air in the exhaust gas stream except for the catalytic removal of NO by CO.

It has also been found that many of the desirable catalysts for effecting the oxidation and treatment of the waste product stream for improving it, including the reduction of NO by CO, prior to discharge into the atmosphere, are harmfully affected by lead or lead compounds, minor components of the gasoline including phosphorus, sulfur and lead compounds.

Similarly, other compounds from waste industrial gases, other than auto or engine exhaust streams may have exerted a poisoning effect on a catalyst bed and it is an advantage to device a system whereby no catalytic material is needed to initiate the removal of NO, from the streams.

It has previously been thought that chemical reduction of NO, through addition of fuel into the exhaust system might not achieve reduction in the desired sense of causing it to form N It was the scientific feeling that such reduction would merely convert some of the NO, into undesirable nitrogenous products which are equally objectionable. H. P. Schuchmann and K. J. Laidler suggested in the Journal of the Air Pollution Control Association, Vol. 22, No. l,pages 52 and 53 (January 1972) that the use of hydrocarbons for NO reduction mainly, produces undesirable reduction products. Contrary to this teaching, it has now been discovered that a thermally activated gas phase chemical kinetic reaction can be employed to control NO, emission problems. This method provides an additional advantage in the case of stationary sources in that cheaper and/or more readily available fuels having high nitrogen content can be used, since the extra nitric oxides thus formed from combustion of these fuels, can also be reduced by this method; for example, shale oil, coal and some heavy petroleum fractions.

Briefly, the present invention provides a method for treating a combustion effluent or waste product stream containing NO and other contaminants, said method comprising the steps of first contacting the stream with a sufficient amount of hydrocarbon in the presence of oxygen at a temperature ranging from 400 to 2,700C. for a sufficient time to substantially reduce the NO, to molecular nitrogen; then contacting the reduced stream with sufficient oxygen at a temperature ranging from 400 to 2,700C. for a sufficient time to substantially oxidize all other contaminants to an oxidized state.

It should be noted that the hydrocarbons and substituted hydrocarbons described herein are far more effective and powerful gas-phase, non-catalytic reducing agents for NO, than species such as carbon monoxide (CO) and hydrogen (H which have been found, by the experiments in the apparatus used herewith, to be essentially ineffective at the conditions herein de: scribed. Also, it is important to realize that this present concept of adding hydrocarbon or substituted hydrocarbons and oxygen to the effluent or exhaust stream is a wholly different concept from adding additional fuel to the primary combustion zone, which is simply running fuel rich as it is commonly termed. In the latter case, it is possible to decrease NO by a certain limited amount, by lowering the adiabatic flame temperature and by the concurrent reduction in the oxygen available to form nitrogen oxides due to the excess hydrocarbon present during the actual combustion, but in the present case, the hydrocarbon and oxygen are added after combustion is complete, and any NO, previously formed is chemically reduced under controlled conditions in the effluent stream. This allows the designer to run the engine or burners as he feels is best for the process, rich or lean, which is a most significant advantage. It is also a wholly different concept from afterburning (such as in the John Zink process as described in British Pat. No. 1,274,637) in which another smaller flame is actually maintained downstream of the primary flame by injecting fuel and burning it, in order to incinerate contaminating gases.

The invention may be better understood by reference to the following drawings wherein:

FIG. I is a diagrammatic view of an internal combustion engine employing this invention;

FIG. II is a diagrammatic view of a stationary source employing this invention;

FIG. III is a graphic illustration showing the effect of increasing the oxygen concentration.

In a more specific embodiment of the present invention, there is provided a method and apparatus for treating the exhaust gas of internal combustion engines containing CO, NO and other contaminants, said method comprising first contacting the exhaust gas with a sufficient amount of gasoline, along with a small, carefully adjusted amount of air, at a temperature ranging from 900C. to 1,450C. for a sufficient time to reduce the NO, to molecular N and then contacting the reduced exhaust gas with a sufficient amount of oxygen in air to oxidize the excess hydrocarbon, CO, and small amounts of NH and HCN, to equally small amounts of N and less objectionable products upon their admission to the atmosphere. The small amount of NO thus formed together with the small amount of NO left unreduced, will remain significantly less than maximum acceptable tolerances.

The present invention may be more clearly understood and explained in connection with its use for removing NO, compounds from the exhaust of internal combustion engines when read in conjunction with the accompanying drawing wherein FIG. 1 represents a diagrammatic showing of an internal combustion engine employed in this invention.

Referring to FIG. 1 there is disclosed the general showing of an apparatus or engine system wherein the invention is used; such system consists basically of an internal combustion engine with an exhaust system including an exhaust manifold 11 leading to a heat exchanger 12 which is in connection with a muffler not shown. Valve 13 is the exhaust valve and functions in accordance with normal design characteristics of the engine employing the invention.

The heat exchanger 12 can take the form of any suitable shape such an elongated hollow cylinder. The exchanger is made of any satisfactory structural material such as stainless steel or other high temperature alloy or ceramic. The cylinder is closed at one end 14. A concentric hollow tube 18 extends axially along the inside of the cylinder to define the inner reactor passageway l8 and the concentric, exit passageway 16 and 25. Passageway 18 connects with the engine cylinder through valve 13 so that the exhaust gas passes therethrough. Passages 16 and 25 allow for the return of the treated exhaust gases so that their heat is taken advantage of in passage 18. Passageway l6 and 25 connect with the muffler (not shown) through passageway 22.

The hydrocarbon injection can take place through a fuel source which in the case of automobiles can be the original source of power for the engine which then is passed via line 24 to a carburetor 26. The carburetor is then activated by an automatic throttle control 28 to pass, into the passage 18, sufficient fuel and air to cause reduction of the NO, components contained in the exhaust gas.

In addition, excess air is interjected into passage 18 at the lower end thereof, to account for the oxidizing of excess hydrocarbon as well as any carbon monoxide and other oxidizable contaminants contained in the fuel.

The concept has been tested in a commercial furnace, a schematic of which is shown in FIG. 2 of the accompanying drawing. This furnace burns any regular fuel oil (such as No.6) which enters furnace at 30, with air added at 32. The principal flame occurs at the main combustion area 34 and the heat is usually used to heat a boiler. The nitric oxide formed is carried downstream in the effluent gas 36, where it is met and mixed with the necessary mixture of a hydrocarbon and nitrogen. Air (oxygen) is not needed with the hydrocarbon in this case, since the furnace effluent generally carries enough excess oxygen downstream 36, to provide the correct ratio of [Ofl/[C]. The hydrocarbon issues forth from a water-cooled probe at points 38 and 40, and is supplied to the probe at 42. The watercooling occurs in a jacket around the inner tube, through 44, 46 and 48. Cheaper burner fuels containing high nitrogen contents which produce more NO, can be used here, by merely increasing appropriately, the flow-rate of hydrocarbon added. The unburned hydrocarbon, CO and any small amounts of HCN and NH are oxidized at a final stage by adding air at 50 and 52 at the elevated temperatures of the effluents. The clean effluent finally exits at 54 and 56. The [O ]/[C] ratio must be maintained greater than 1.0 at a range of from 800C. to 2,000C. and preferably from 800C. to l,700C.

Any hydrocarbon, as well as oxygen-containing hydrocarbons, sulfur-containing hydrocarbons, nitrogencontaining hydrocarbons, halogen-containing hydrocarbons, which yield carbon containing free radicals on pyrolysis under the conditions shown can be employed in this concept. In the case of a fuel operated mobile source, the fuel itself such as gasoline, would be the most convenient and effective reducer. Suitable hydrocarbons include methane, ethane, the propanes, butanes, pentanes, hexanes, heptanes, octanes, nonanes, dodecanes, as well as aromatic compounds consisting of benzene, naphthalene, anthracene, toluene, mesitylene. Suitable oxygenated hydrocarbons include alcohols, ethers, acids, ketones end aldehydes.

There are two critical ratios which are used in this method to optimize the favorable reduction of NO. One is [C]/[NO,] which is the total number of carbon atoms per mole of NO supplied by the hydrocarbon, CnHm, wherein n ranges from 1 to 12 and m is any integer corresponding to an existing compound, and the other is [G l/[C] (this follows as the quotient, of the ratio [OJ/[NO] mentioned in the previous filing and [Cl/[NO] Where there is no excess oxygen present in the combustion effluent, the ratio of C ]/[NO,] ranges from 0.02 to 400. Preferably, the ratio of C]/[N0,] ranges from 4:1 to 32:1. lt is to be understood that the lowest limit would be the stoichiometric amount of hydrocarbon necessary to form carbon monoxide, nitrogen and other products with the nitrogen oxides. The ratio [C l/[C1 can vary between 0.01 and 12. in general. Preferably, the ratio of [O lllCl ranges from 0.25 to l with optimum NO-reduction performance between 0.5 and 0.8. When carbon monoxide, CO, is present in the effluent, the amount of oxygen must be increased to oxidize the CO to CO 1f excess oxygen is already present in the combustion effluent, then sufficient hydrocarbon or other reductant must be added to provide the desired optimum [C l/[C] ratio. It is to be understood that any of the oxygenated hydrocarbons, as well as the other compounds containing heteroatoms would operate within the same ranges as the hydrocarbons. However, it is to be understood that any carbon atoms bound to oxygen in such compounds are not considered in the carbon to oxygen ratio. H I g v V The temperature for the reduction step ranges from 400C. to 2,700C. and preferably from 700C. to 2,200C., more preferably from 700C. to 1450C. in the exhaust from an internal combustion engine. Because of the chemical kinetics, the temperature which is used is greatly influenced by the [C]/[NO,] ratio and by the residence time in passageway 18. The residence time in passageway 18 ranges from 25 to 300 milliseconds, preferably from 100 to 250 milliseconds.

This reaction can be carried out over a wide range of pressures, i.e., 0.1 to 100 atmospheres, preferably l to 40 atmospheres. The pressures are not critical and the chemical kinetics are adjusted to fit these conditions. The velocities of the combustion effluent as well as the mixing of the hydrocarbons in the postcombustion zone are regulated so that there is a sufficientresidence time to enable the hydrocarbon to reduce the NO from the combustion effluents. Under most circumstances, operating at higher pressure requires less residence time.

it has also been observed that when the hydrocarbon and oxygen are turned off, reduction of the NO continues for a time thereafter. This continuous reduction lasts for as long as several hours as a result of hydrocarbon air addition lasting only a small fraction of that time (e.g., minutes) and is called the Self-Contained Reductant Mode, since the reductant exists on the walls of the stream tube. This effect is demonstrated in runs 7 and 8. Sufficient wall area relative to the main flow is required for this to take place.

EXAMPLE 1 A typical exhaust gas containing 1000 ppm NO, 13 percent CO and 1 percent CO flows through the exhaust manifold 11 and reducing section 18 at an average temperature of 1,227C. after leaving the exhaust valve 13. By allowing its residence time at this average temperature to total milliseconds after introduction of 0.1 percent isobutane and 0.25 percent 0 (0.25percent already present) (since the concentration of NO is 0.1% and the ratios [C]/[NO] and [O I/[NO] are 4 and 5, respectively), the concentration of NO will be reduced to 50 ppm. Any small, residual amounts of contaminants are oxidized in the Oxidizer Reactor Section, before passing into the muffler. This example is shown in the second case of Table 1.

Other examples in Table I illustrate the effects of various parameters on the reducing ability of this process. By increasing both ratios [Cl/[NO] and [Ofl/[NO], for example, by a factor of 3.3., NO is reduced in Case 2 to 50 as compared to 270 ppm in 1. in Case 3, a reduction of 100C. in temperature causes [NO] to rise. 100 ppm. By increasing the [Cl/[NO] ratio and decreasing the [O in Case 4, [NO] increases to 650 ppm, but then m Case 5, increasing t he [OJ/[N01 ratio frorn 0 to 7 causes [NO] to drop to ppm. Case 6 illustrates the effect in a mixture with no CO or CO, in which it is seen that a higher temperature is required when no CO or CO is present.

Cases 7 and 8 are examples of an effect newly discovered in this work, in which it has been found that a reductant consisting of a hydrocarbon-like material, is laid down on the reactor wall for a period of time, e.g., 5 or 10 or more minutes, by flows of iC,H and 0 such as those listed in 7 and 8. When the iC H and 0 are turned off, reduction of the NO continues for one-half hour and 1 hour, respectively. This has been named the Self-Contained Reductant Mode (SCR Mode) and is quite effective as seen from the [NO] level maintained, l0 and 70-230 ppm, respectively. These cases are mixtures without CO or CO. With CO and CO present, lower temperatures are required to obtain similar effects with the SCR Mode.

Case 9 shows the effect of lengthening the residence time and optimizing the proportions of iC H and O in order to decrease the reaction temperature.

Case 10 illustrates the use of an oxygenated hydrocarbon with this reduction process. In this situation, acetone is used instead of isobutane.

Case 11 demonstrates the application of a commercial-type gasoline, Exxon Plus, in place of the isobutane.

TABLEl Reduction of Nitric Oxide (NO) by lsobutane NO in ppm prior (for H2 hr.)

TABLE I -Continued Reduction of Nitric Oxide (NO) by lsobutane NO in ppm prior to iC H Addi- NO in ppm Residence Case tron (Bal. lS N with iC H [C] Temp. Time No. in all cases) addition [NO] [NO] C. Millisec 8 I000 70 to 230 0 (l6) 0" (4) 1240 100 i (for 1 hr.) 9 MOO-H392 CO +l7( CO 390 4 I0 830 174 [COW-H37: CO +l7z CO 390 76 7 1027 174 ii l0OO"+l37r CO +l7c CO 70" 8 10 1135 100 "Intermittent mode: iC H and O, are shut off. after reaction tube has been conditioned with the ratios of iC H and 0, indicated in parentheses, for l0-l5 minutes. Water vapor is formed and thus present in all these reactions in amounts roughly equal to the N0 concentration. Acetone was used here instead of iC H for the reduction process. Some minor fraction of the NO may have been catalytically reduced by CO. ""Exxon Plus gasoline" was used here instead of isobutane.

As indicated previously, the combined addition of 7' TABLE III i ai w' oxygen (Q n the form of r, along ith the hydrocar Temp (,C.) 967 H02 H12 H77 247 bon is vital to the process minimizing both NO and iCJ-l Reacted 0 62 59 93 98 HCN. First, shown below in Table [I are examples of NO Ream-d O 18 100 100 the effect of side product formation, viz. HCN and EXAMPLE 2 NH After a final stage of oxidation, the total NO allowed to pass into the air would be the sum of NO remaining after reduction plus NO from oxidation of 25 In a commercial Cleaver-Brooks Furnace, No. 6 Fuel Oil enters the furnace and air is added thereto. The principal flame occurs at the main combustion area and HCN and NH the heat is used for heating a boiler. The nitric oxide fo- TABLE II NO in ppm prior to iC H addi- Res. NO in ppm with Case tion (Bal. is N; Temp. Time [C] [0 iC H addition No. in all cases) C. msec l lO]' [NO] [NO] [HCN] [NH 12 1000 1227 I00 4 5 12 44 13 1000 1277 100 48 32 75 28 I4 1000 910 I 200 40 3O 14 I5 1000 I200 200 40 30 lo Second, in the accompanying graph of FIG. 3 are shown data which illustrate how increasing oxygen partial pressure actually decreases the amount of HCN present in the effluent under the conditions described.

Additional data corresponding to very short residence times (1 to 2 millisec) and high pressures (13 atmospheres) have also been obtained using a single pulse shock tube, which show that NO is reduced completely by iC I-l by l,l77C. as shown below in Table III for a mixture of 0.2% NO and 1.0% iC H in argon.

rmed is carried downstream in the effluent gas where it is mixed with a mixture of propane and nitrogen. Air (oxygen) is not needed with the propane since the furnace effluent generally causes enough excess oxygen downstream to provide the correct ratio of [O ]/[C]. The propane is injected into the furnace through watercooled probes. The unburned hydrocarbon, CO and any small amounts of HCN and NH are oxidized at a final stage by adding air at the elevated temperature of the effluents, e.g., ll0OC. The clean effluent finally exits from the furnace.

Data obtained are shown in Table IV.

TABLE IV REDUCTION OF NITRIC OXIDE BY C H IN CLEAVER-BROOKS FURNACE "Oxygen (0,) always at 3% excess, based on fuel oil only. One foot of cooling allowed to yield this temperature. "Each of these experiments represents a higher degree of mixing than its intermediate predecessor. by the use of increased nitrogen diluent.

What is claimed is:

l. A thermal method for treating a stream of combustion effluents issuing from either a mobile or stationary source containing NO and other contaminants; said method comprising the step of injecting into said stream a hydrocarbon compound and oxygen in sufficient amounts to maintain the ratio of [C] to [No no greater than 32 to l and the ratio of oxygen molecules to carbon atoms no greater than 2.5 to l, said hydrocarbon compound being one selected from the group consisting of hydrocarbons, oxygen-containing hydrocarbons, nitrogen-containing hydrocarbons, halogencontain'ing hydrocarbons, and sulfur-containing hydrocarbons at a temperature ranging from 400 to 2,700C for a sufficient time to substantially reduce the NO, to molecular nitrogen.

2. A thermal method for treating the exhaust gas of internal combustion engines containing CO, NO, and other contaminants, said method comprising the steps of injecting into the exhaust gas, gasoline and air in sufficient amounts to maintain the ratio of [C] to [N no greater than 32 to l and the ratio of oxygen to carbon atoms in a range no greater than 2.5 to l at a temperature ranging from 700C to l,450C for a sufficient time to reduce the NO, to molecular nitrogen and then contacting the reduced exhaust gas with a sufficient amount of oxygen to oxidize the excess hydrocarbon, CO and other oxidizable contaminants to less objectionable products upon emission into the atmosphere.

3. A method according to claim 2 wherein the residence time ranges from 25 to 250 milliseconds.

4. A method according to claim 1 wherein said hydrocarbon is a commercial grade gasoline 5. A method according to claim 1 further including the step of intermittently contacting the effluent stream with said hydrocarbon and oxygen.

6. A thermal method for treating the effluent gas from a stationary source containing CO, NO, and other contaminants, said method comprising the steps of injecting into said effluent a hydrocarbon and air, in sufficient amounts to maintain the ratio of [C] to [NO,] no greater than 32 to l and the ratio of oxygen molecules to carbon atoms no greater than 2.5 to l, at a temperature ranging from 700C to 2200C for a sufficient time to reduce the NO, to molecular nitrogen; contacting the reduced effluent gas with a sufficient amount of oxygen to oxidize the excess hydrocarbon, CO and other oxidizable contaminants to less objectionable products upon admission to the atmosphere.

7. A method according to claim 6 wherein the ratio of O2/C in the second contacting is greater than I.

l i= l =l= 

1. A THERMAL METHOD FOR TREATING A STREAM OF COMBUSTION EFFLUENTS ISSUING FROM EITHER A MOBILE OR STATIONARY SOURCE CONTAINING NOX AND OTHER CONTAMINANTS, SAID METHOD COMPRISING THE STEP OF INJECTING INTO SAID STREAM A HYDROCARBON COMPOUND AND OXYGEN IN SUFFICIENT AMOUNTS TO MAINTAIN THE RATIO OF (C) TO (NOX) NO GREATER THAN 32 TO 1 AND THE RATIO OF OXYGEN MOLECULES TO CARBON ATOMS NO GREATER THAN 2.5 TO 1, SAID HYDROCARBON COMPOUND BEING ONE SELECTED FROM THE GROUP CONSISTING OF HYDROCARBONS, OXYGEN-CONTAINING HYDROCARBONS, NITROGEN-CONTAINING HYDROCARBONS, HALOGENCONTAINING HYDROCARBONS, AND SULFUR-CONTIANING HYDROCARBONS AT A TEMPERATURE RANGING FROM 400* TO 2,700*C FOR A SUFFICIENT TIME TO SUBSTANIALLY REDUCE THE NOX TO MOLECULAR NITROGEN.
 2. A thermal method for treating the exhaust gas of internal combustion engines containing CO, NOx and other contaminants, said method comprising the steps of injecting into the exhaust gas, gasoline and air in sufficient amounts to maintain the ratio of (C) to (NOx) no greater than 32 to 1 and the ratio of oxygen to carbon atoms in a range no greater than 2.5 to 1 at a temperature ranging from 700*C to 1,450*C for a sufficient time to reduce the NOx to molecular nitrogen and then contacting the reduced exhaust gas with a sufficient amount of oxygen to oxidize the excess hydrocarbon, CO and other oxidizable contaminants to less objectionable products upon emission into the atmosphere.
 3. A method according to claim 2 wherein the residence time ranges from 25 to 250 milliseconds.
 4. A method according to claim 1 wherein said hydrocarbon is a commercial grade gasoline.
 5. A method according to claim 1 further including the step of intermittently contacting the effluent stream with said hydrocarbon and oxygen.
 6. A thermal method for treating the effluent gas from a stationary source containing CO, NOx and other contaminants, said method comprising the steps of injecting into said effluent a hydrocarbon and air, in sufficient amounts to maintain the ratio of (C) to (NOx) no greater than 32 to 1 and the ratio of oxygen molecules to carbon atoms no greater than 2.5 to 1, at a temperature ranging from 700*C to 2200*C for a sufficient time to reduce the NOx to molecular nitrogen; contacting the reduced effluent gas with a sufficient amount of oxygen to oxidize the excess hydrocarbon, CO and other oxidizable contaminants to less objectionable products upon admission to the atmosphere.
 7. A method according to claim 6 wherein the ratio of O2/C in the second contacting is greater than
 1. 